Sabres coach Dan Bylsma announced the diagnosis after Sunday's practice, saying that an MRI revealed the nature of the injury.
Lehner suffered the injury while making his Sabres debut in Thursday's loss to the Ottawa Senators.
Chad Johnson, who started Saturday's game against Tampa Bay, is expected to remain Buffalo's starting goaltender while Lehner is sidelined.
The Sabres acquired Lehner and center David Legwand from Ottawa last summer in exchange for the 21st overall pick in June's draft.
Information from The Associated Press was used in this report.